What’s Up With That: Beer in a Hot Pan Slides Around Like an Air Hockey Puck

Time for a kitchen experiment. Take a beer and dump it in a hot pan. It will probably boil furiously away. But if you crank up the flame under the pan and wait for it to get really hot before pouring on the beer, something completely different will happen.

What’s Up With That

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The beer (or water if you don’t want to waste a good beer) will turn into a blob that doesn’t quickly boil away, instead sliding around on the pan like an ice skater over ice. Responsible for this unexpected turn of events is a physical phenomenon known as the Leidenfrost effect, and it might one day help you walk over hot coals or dip your hand in molten lead without getting burned. (Note: WIRED does not condone doing either of these tricks ever; you will probably hurt yourself.)

What’s Up With That?

The Leidenfrost effect was first described by Johann Gottlob Leidenfrost, an 18th century German doctor whose name falls somewhere between physicist Omar Hurricane and engineer Isambard Kingdom Brunel on the Scientist-Name-Badassness-O-Meter. In “A Tract About Some Qualities of Common Water,” Leidenfrost described an experiment where he pulled an iron spoon from a fire and then dribbled water on it. He found that the first water droplet would last around 30 seconds, a second drop would last 10 seconds, and subsequent drops would quickly disappear.

(For the sake of completeness, I should mention that Victorian boiler-maker Sir William Fairbairn later investigated this strange effect in more detail, citing the work of Pierre Hippolyte Boutigny, a man who basically breaks the Scientist-Name-Badassness-O-Meter.)

The Leidenfrost effect occurs when a drop of liquid comes in contact with a surface significantly hotter than its boiling point. According to the idea that stuff on a hot surface boils away fast, it would seem to make sense that stuff on a really hot surface boils away really, really fast. But actually, the opposite is true. Put a drop of water, which has a boiling point of 100 degrees Celsius (212 degrees Fahrenheit), on a surface with a temperature at least 200 degrees Celsius (392 degrees Fahrenheit) and its bottom will instantly vaporize. This forms a layer of vapor underneath the drop, hoisting the rest of the liquid above it. Because water vapor conducts heat an order of magnitude less efficiently than liquid water, the drop becomes insulated against the raging heat below it. Instead, it floats on this cushion of vapor like a puck above an air hockey table.

As the vapor steams away (it’s a gas after all) the bottom of the liquid is still feeling the heat and producing more vapor, which replenishes the insulating layer. Even still, this cushion can’t protect the water forever. It simply boils away at a much slower rate than if you had put it on a surface that wasn’t as hot.

Now that you know about the Leidenfrost effect, you could try the beer experiment yourself. A slightly more awesome/dangerous thing that people have done is stick their hand in molten lead and keep all their fingers. This crazy idea occurred to physicist Jearl Walker of Cleveland State University, who remembered a late 19th-century carnival trick where a performer was reportedly able to dip wet fingers into molten lead. Walker hypothesized that the water touching the lead (heated to around at least 328 degrees Celsius) would form an insulating vapor layer (.pdf) allowing the performer to very briefly escape being burned.

Walker performed his own experiment to test this out, melting some lead and then wetting his hand with tap water. He tried to briefly touch the surface of the lead with his wet hand.

“I must confess that I had an assistant standing ready with first-aid materials,” wrote Walker, the science daredevil. “I must also confess that my first several attempts failed because my brain refused to allow this ridiculous experiment, always directing my finger to miss the lead.”

Eventually, he was able to overcome rational thought and push his finger quickly in and out of the lead. He claims to have felt no heat. He repeated this several times, at one point managing to dip all his fingers in at once and touch the bottom of the pot. Please watch the video at right to see Walker repeating the presentation and explaining many of its dangers. And again, please, don’t try this at home. Walker is a trained science professional.

If you’ve ever been to a scientific laboratory, you might have also seen the Leidenfrost effect when a grad student friend knocked over a Dewar flask of liquid nitrogen, which boils at −196 degrees Celsius. Because the floor’s temperature is much, much hotter than this, the liquid nitrogen will turn into tiny air hockey pellets and skitter around everywhere. (One final warning here that liquid nitrogen will cause freeze burns so don’t touch it with bare skin.)

And if you’re really clever, like students Carmen Cheng and Matthew Guy of the University of Bath, you might build a Leidenfrost maze. Cheng and Guy realized that a hot surface with a saw-tooth texture will propel Leidenfrosting water droplets in a particular direction. Their creation, the Leidenfrost maze, deflected the drops around like rolling marbles. You can see a video of this fun experiment below.

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